Rotary actuator device



Aug. 23, 1966 a. H GRAHAM ROTARY ACTUATOR DEVICE 2 Sheets-Sheet 1 Filed May 15, 1962 A. m f

.M L. M M R NG VH m L R v A 4 C ATTORNEY 3, 1966 c. H. GRAHAM ROTARY ACTUATOR DEVICE 2 Sheets-Sheet 2 Filed May 15, 1962 INLET EXHAUST I I I INVENTOR. CHARLES H. GRAHAM LWQML United States Patent 3,267,816 ROTARY AQTUATOR DEVICE Charles H. Graham, Mountain View, Calif., assignor to Graham Engineering Company, Inc., Palo Alto, Calif., a corporation of California Filed May 15, 1962. Ser. No. 194,924 11 Claims. (Cl. 91-178) This invention relates generally to rotary actuators, and more particularly to a rotary actuator which provides output shaft rotation in response to the application of an actuating fluid such as pressurized hydraulic or pneumatic fluids.

This invention concerns certain improvements in actua' tors of the general type disclosed in my copending application Serial No. 111,017, filed May 18, 1961, for Power Translator, now Patent No. 3,121,371. The power translator described in my copending application utilized a large and a small double acting piston disposed in adjacent and substantially parallel cylinders which are communicated, at respective adjacent end portions, by opposite chambers. Each chamber includes .a rotary mounted sprocket and a chain mounted on the sprocket and interconnecting adjacent ends of the double acting pistons, thereby connecting the pistons in series with one another.

In the operation of the power translator described in my copending application, one of the opposite chambers is pressurized with respect to the other by the actuating fluid so that identical pressure ditferentials are established across both pistons urging the pistons in the same direction. Since the pistons are connected in series and are of different size, the net effective force actuating the system is equal to the pressure differential times the difference in the area of the individual pistons. As fully explained in my copending application, the smaller piston is selected to be as small as possible so that the effective area (difference between the individual piston areas) is as large as possible yet of suflicient diameter to seal the chain communication channel. Rotation of the output shaft is provided by keying one or both of the rotary mounted Sprockets to their respective mounting shafts.

While this invention is very useful for providing actuator fluid controlled rotary motion, there are certain applications in which an increase of rotary output power for a given diameter of the power piston may be found desirable. Also, in those applications where only a single output shaft is utilized, certain simplifications of construction may be achieved by omitting one of the sprockets and its associated suspension system. The omitting of one of the sprockets also does away with one of the chambers and the tensioning means for keeping the chain taut.

It is therefore a primary object of this invention to provide a rotary actuator device which is simple in construction, reliable in operation, and has a high power conversion efficiency.

It is another object of this invention to provide a rotary actuator device having an output torque which is proportional to the operating pressure and the area of one of the cylinders.

It is a further object of this invention to provide a simpiified rotary actuator having a single output shaft for coupling to a utilization device and which is operable with a pressurized actuating fluid such as a liquid or .a gas.

It is a still further object of this invention to provide a rotary actuator which may be constructed to have high or low torque outputs and slow or fast shaft rotations either larger or smaller than one revolution.

It is still another object of this invention to provide a rotary actuator for converting hydraulic or pneumatic pressure differentials to rotary motion.

Briefly, in a preferred embodiment of this invention, a pair of cylinders are disposed side by side in parallel relation. A single chamber is utilized to interconnect adjacent ends of the cylinders and a chain sprocket is rotatably mounted upon an output shaft which passes through and is rotatably mounted by the chamber walls. The center to center separation between the cylinders is selected to be substantially equal to the chain sprocket pitch diameter.

A double acting piston is fluid-tightly slidable in each cylinder. The pistons are interconnected by the chain which passes from one piston, over the sprocket in the chamber, to the other piston. The cylinder ends remote from the chamber, are sealed but either is connectable with a vent while the other is connecta'ble with a source of actuating fluid under pressure. Actuating fluid is introduced simultaneously into the chamber (and the cylinder portions in communication with the same) and one sea'led cylinder end portion while the other sealed cylinder end portion is vented downstream.

In this manner, one of the double acting pistons is exposed to pressurized actuating fluid on both sides so that the net force acting thereon is zero. The other double acting piston is exposed, on one side, to the actuating fluid in the chamber (upstream) and on the other side to the lower downstream pressure so that said other piston moves in the downstream direction thereby rotating the output shaft. The system is reversed by venting the previously pressurized closed cylinder end portion downstream and pressurizing the previously vented closed cylinder end portion with actuating fluid.

Other objects and a better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is an elevational side view of the rotary actuator device of this invention;

FIG. 2 is .a cross-sectional view taken along line 2-2 of FIG. 1;

FIG. 3 is a side view, partly in section and partly in elevation, of the rotary actuator device of FIG. 1;

FIG. 4 is a cross-section view taken along lines 44 of FIG. 1;

FIG. 5 is a schematic flow diagram of the actuator fluid control system for actuating the rotary actuator device of FIG. 1;

FIG. 6 is a side view, partly in section and partly in elevation, similar to FIG. 2, showing certain modifications which may be incorporated in the rotary actuator device of this invention; and

FIG. 7 is a schematic flow diagram of an actuator fluid control system for actuating the rotary actuator device of FIG. 6.

Referring now to the drawings, and particularly to FIGS. 1, 2, 3 and 4, there is shown a rotary actuator device 10 constructed in accordance with this invention and comprising a first cylinder 11, a second cylinder 12, and a housing 18 having a pressurizable and fluid-tightly sealed chamber 13 communicating adjacent end portions of cyiinders 11 and 12. Rotary actuator 10 also comprises a chain sprocket 14 rotatably mounted in chamber 13 and a pair of double acting pistons 15 and 16 slidingly received by and fluid-tightly sealed to cylinders 11 and 12 respectively. A chain 17 engages chain sprocket 14 and is fastened to adjacent ends of double acting pistons 15 and 16 to effectively connect the pistons in series.

Housing 18 may comprise a tubular or hollow body member 20 having flat end faces 21 and 22. Body member is preferably, but not necessarily, selected of rectangular outer cross section primarily to provide a pair of parallel and opposite walls 23 and 24 of suflicient thickness to accommodate bearing assemblies 25 and may be selected from extruded aluminum stock. A closure plate 26 may be bolted to flat end face 21 as shown, by

means of a plurality of fastening bolts 27, engaging threaded dead-ended tapped bores 28. To assure a sealing fluid-tight sealing contact between body member 20 and closure plate 26, preventing leakage of the actuating fluid, an undercut 29 may be provided in body member 20 to form a recess for accommodating a resilient seal means, such as a conventional O-ring seal 30.

Bolted or otherwise aifixed to the other side of body member 20 is an intermediate or transition plate 31 which is fluid-tightly sealed to flat face 22 of body member 20. The fluid-tight seal is provided by means of an undercut 32 accommodating a resilient sealing means, such as conventional O-ring seal 33. Intermediate plate 31 includes a pair of openings 34 and 35 and a pair of bottomed cylindrical bores 41 and 42, axially aligned with openings 34 and 35, for receiving the end portions of cylinders 11 and 12. A further closure plate 36, including a pair of short bottomed cylindrical bores 37 and 38, is provided for receiving the other end portions of cylinders 11 and 12. Closure plate 36 is clamped to body member 20 by means of a plurality of threaded clamping bolts 39 engaging threaded bores 40. In this manner, closure member 36 firmly clamps cylinders 11 and 12 and intermediate plate 31 against body member 20. Intermediate plate 31 also forms a transition between chamber 13 and the space inside cylinders 11 and 12.

To provide a fluid tight seal between intermediate plate 31 and the adjacent end portion of cylinders 11 and 12, a substantially rectangular recess 44 is formed in the outer peripheral surface of cylinders 11 and 12 (or the inner peripheral surfaces of bores 41 and 42) for accommodating resilient seal means such as conventional O-ring seals 45. Likewise, bores 37 and 38 on the other end portion of cylinders 11 and 12 are provided with peripheral recesses 44 for receiving resilient sealing means such as conventional O-rings 45.

Rotatably supported within chamber 13 is chain wheel or sprocket 14 integrally mounted to an output shaft 50 by means of a pin 51 (or some other suitable means such as a key) as best seen in FIG. 2. To rotatably support output shaft 50, a pair of axially aligned bearing openings 52 and 53 are provided in side walls 23 and 24. Each opening is lined with a bearing retainer ring or bushing 54 and 55, having outer shoulders seating against the inner surface of walls 23 and 24 respectively and inner shoulders for seating a pair of pin bearings 56 and 57 respectively. A plastic expandable U-shaped type of sealing member 58 is seated in the inn-er end portion of each bearing retainer ring 54 and 55 against pin bearings 56 and 57 and faces upstream pressure with its open end. A pair of flat thrust washers 59 may be flush mounted to the end portion of the bearing retaining rings 54 and 55 and a spacer ring '60, dimensioned to prevent lateral motion of. chain sprocket 14, which may be utilized to fill the remainder of the space between bearing retaining rings. In this manner, chain sprocket 14 is mounted in chamber 13 rigid with output shaft 50 is rotatably mounted in body member 20.

As best shown in FIG. 3, each double acting piston and 16 is connected to chain 17 which passes over chain sprocket 14. For proper alignment of chain 17, chain sprocket 14 is selected so that its pitch diameter is substantially equal to the center-to-center separation of cylinders 11 and 12. In this manner, the portions of the chain between chain sprocket 14 and pistons 15 and 16 respectively are always parallel and the pull on or by pistons 15 and 16 is always axial.

Even though cylinders 11 and 12 of actuator 10 are shown as being of substantially equal diameter, it is within the contemplation of this invention to utilize cylinders which are of different diameters as will hereinafter be explained more fully. As will become better understood hereinafter, as long as cylinders 11 and 12 are of the same diameter, actuator 10 of this invention provides a '72 and is free to move, under output rotation of substantially the same angular velocity in the reversed or forward direction for the same actuating fluid velocity. Selecting cylinders 11 and 12 of different diameter provides an actuator which, for the same velocity of actuating fluid, provides either a fast forward and slow reverse rotation, or a fast reverse and a slow forward rotation at the expense of the output torque.

Double acting pistons such as pistons 15 and 16 may be of any conventional form. However, these pistons are perferably constructed of a pair of expandable disk like members 60, 61 of thin metal or plastic having a bent rim portion and mounted back-to-back between two stiff backing members 62, 63 clamped together by a bolt and nut means 64. More particularly, disk like member is spun, clamped against a backing plate, on a lathe or similar turning device and the edge is spun swaged over the backing plate. The term spun swaged is used herein to denote the operation of simultaneously turning and bending. As long as disk like member 60 is sufficiently thin and resilient and faces upstream in a cylinder, the pressure urges the outward turned rim portion into intimate sealing contact with the cylinder wall. Backing plates 62 and 63 provide axial directed stiffness and prevent deformation in the direction of the cylinder axis. It has been found that a convenient method of spin swaging the edge is to apply pressure, during spinning, with a flat instrument. Once the edge is spun swaged, it takes on a resilient set which lasts indefinitely. Closure plate 36 is provided with a pair of threaded openings for engaging a pair of pipe fittings and 71 to which auxiliary piping, carrying actuating fluid, may 'be connected. Also mounted to closure plate 35 is a shuttle valve 72 which may be of conventional construction. Closure plate 36 is provided with a pair of input ports '73 and 74 and an output port 75 each of which is extended into the shuttle valve to communicate with an axial body space in the valve. A floating ball member 76 is located in the axial body space of shuttle valve pressure, between body valve seats 77 and 78 for respectively sealing input ports 73 and 74. The port 73 communicates with cylinder 11, the port 74 communicates with cylinder 12 and the output port 75 communicates with a pipe fitting 79 to which a pipe 80 is connected. The other end of pipe 50 is connected to a pipe fitting Which communicates through a passage 91, through intermediate plate 31 with chamber 13.

Referring now particularly to the FIGURE 5 diagram,

- it will be seen that the flow of fluid to and from the cylinders 11 and 12 can be controlled by a conventional four way control valve having a movable valve element 100, an inlet port connecting with a pressure fluid supply pipe 101, and outlet port connecting with an exhaust pipe 102, and a pair of service ports connecting with output or service pipes 104 and 105 that are respectively connected to the cylinder fittings 71 and 70. The valve element of a control valve of this type is movable to each of a pair of operating positions to communicate its inlet (pipe 101) with a selected one of its service ports (pipe 104 or and to communicate the non-selected service port (pipe 104 or 105) with its outlet (exhaust pipe 102). The valve element 100 of the control valve can be shifted either manually or by well known automatically operated means between one operating position shown, and a second operating position indicated by the broken line 103. In addition, the valve element 100 of such control valves are ordinarily movable to a neutral or hold position, not shown, at which it functions to close off both service ports (pipes 104 and 105) from both its inlet (pipe 101) and its outlet (pipe 102).

It should be noted that rotary actuator device 10 is connected to valve 100 by only two pipe lines 104 and 105 for forward and reverse rotation of output shaft 50 because chamber 13 is pressurized through shuttle valve 72. In certain applications in which there is a large physical separation between rotary actuator device and valve 100, this arrangement represents an important advantage. As will be explained in connection with the control diagram of FIG. 7, in the absence of a shuttle valve a third pipe connection becomes necessary.

Operation of rotary actuator 10 is best explained by first assuming that the valve element 100 is set to one operating position shown in FIG. 5 in which it communicates inlet pipe 101 with outlet pipe 104 and pipe 102 With pipe 105. High pressure actuating fluid is injected from supply line 101 to pipe 104 and into the lower cylinder portion of cylinder 12. The lower cylinder portion is arbitrarily defined as the cylinder space on the side of the piston facing plate 36 and upper cylinder portions is defined as the cylinder space on the side of the pistons facing chamber 13.

Through passage 74, the high pressure actuating fluid also enters shuttle valve 72 and causes freely floating spherical valve element 76 to move over against valve seat 77 to thereby seal passage way 73. Actuator fluid therefore passes through shuttle valve outlet passage and through line 80 to chamber 13 filling it completely and also both upper cylinder portions. Consequently, only the lower portion of cylinder 11 does not receive any actuating fluid and remains in communication with the atmosphere or downstream pressure through pipe 105 and exhaust line 102.

Accordingly, double acting piston 16, being exposed on both sides to the pressurized actuating fluid, has a zero pressure differential thereacross and there is no net force urging it to move. On the other hand, double acting piston 15, being exposed to pressurized actuating fluid on its chamber side and downstream pressure on the other side, has a pressure differential thereacross and there is a net force in the direction of the lower cylinder portion causing it to move, moving with it piston 16 connected thereto by chain 17. Since chain 17 is of unvarying length and since both pistons 15 and 16 are of equal diameter, motion of pistons 15 and 16 will not change the total volume of chamber 13 and the upper cylinder portions so that actuating fluid will merely be transferred from one upper cylinder portion to the other. No fluid is added or removed from the chamber. The angular velocity of output shaft 50 will be substantially proportional to the volume per unit time flowing into the lower portion of cylinder 12.

Upon moving valve element to its second operating position so that inlet and exhaust lines 101 and 102 are respectively connected to lines 105 and 104, pressurized actuating fluid will enter the lower cylinder portion of cylinder 11 and actuating fluid from lower cylinder portion of cylinder 12 will be free to exhaust therefrom. Also, pressurized actuating fluid will cause floating ball valve element 76 to move against the valve seat 7 8 thereby communicating chamber 13 with inlet pressure. This reverses the situation described above so that a net force will be acting on piston 15 to reverse the direction of output shaft rotation.

If cylinders 11 and 12 are selected of different diameters, the Volume of chamber 13 and the upper cylinder portions varies with the position of pistons 15 and 16 requiring means to either supply or to remove actuating fluid therefrom during operation. Supplying actuating fluid is provided automatically by shuttle valve 72. Removing actuating fluid is also provided automatically by shuttle valve 72. As actuating fluid is applied to the lower portion of the smaller piston, shuttle valve 72 supplies the additional Volume needed to keep the upper portion of the larger cylinder filled. Conversely, as actuating fluid is applied to the lower portion of the larger cylinder, shuttle valve 72 permits excess fluid from the upper portion of the larger cylinder to be transferred to the lower portion of the larger cylinder.

Referring now to FIG. 6 there is shown a rotary actuator which illustrates a some what different embodiment of the rotary actuator of this invention. The major difference between rotary actuator 110 and rotary actuator 10 is that the former is provided with an inlet passage directly to chamber 13 and dispenses with shuttle valve 72 and pipe 80. The various parts and components of actuator 110 shown in FIG. 6 which are identical or substantially identical to the parts and components of actuator 10 have been given the same reference characters. Accordingly, as to those parts, the description above is applicable.

Closure plate 112 is similar in all respects to closure plate 26 except for a threaded opening 113 into which pipe fitting 111 is secured for communication with chamber 13. Intermediate plate 114 is also similar in most respects to intermediate plate 31 of FIG. 2 except that pas sage 91 is dispensed with. Closure member 115 is also very similar to closure member 36 of FIG. 2 except for the absence of the passages 73, 74 and 75.

One further modification shown in rotary actuator device 110 is the addition of two expansion springs 116 and 117, extending along the upper cylinder portions of cylinders 11 and 12 respectively. Utilization of springs 116 and 117 are primarily for use with pneumatic actuating fluids such as gases to assure that chain 17 is kept taut in the absence of pressurization. In case of hydrauiic actuating fluids, which are substantially incompressible, there is no real need for springs 116 and 117 since the volume remains constant at all times even in the absence of pressurization.

Referring now to FIG. 7 which shows the schematic control diagram of controlling actuating fluids to rotary actuator 110, either with or without springs, depending on the type of actuating fluid used, it is immediately seen that one additional pipe line connection 116 is necessary to operate actuator 110. The two cylinder connections, namely pipes 101 and 102, are the same as with the rotary actuator 10 shown in FIG. 5, and the third pipe 116 is connected between inlet pipe 101 and fitting 111 on chamber 13. As is immediately seen, hydraulic fluid enters into chamber 13 through pipe line 116 in either operating position of valve element 100, all other connections remaining the same as described in connection with FIG. 5.

As a practical matter, the addition of springs means, such as springs 116 and 117, is desirable for operation with hydraulic or pneumatic fluid to assure that chain 17 is kept under gentle tension in the absence of actuator fluid in chamber 13, a condition encountered during shipping, assembling and cleaning. The springs may be very weak since their only function is to gently urge the pistons towards the lower cylinder portions.

There has been described a rotary actuator device which is simple in construction, reliable in operation, and capable of high torque power outputs. The actuator may be operated with hydraulic actuating fluids as shown in FIG. 3 or with pneumatic actuating fluids as shown in FIG. 6 in which spring means may be utilized to compensate for the fluid compressibility. Operation may be accomplished by use of two pipe lines and a shuttle valve or without a shuttle valve and three pipe lines. The angular rotational limits may be extended by the uses of long cylinders and the torque may be increased by the use of large diameter pistons.

What is claimed is: i

1. A rotary actuator device comprising: a pair of parallel cylinders; a fluid-tightly slidable piston disposed in each cylinder and separating each cylinder into a first and a second cylinder portion; a chamber fluid-tightly sealed across said first cylinder portions and providing communication therebetween; an output shaft rotatably mounted in said chamber; a chain sprocket within said chamber and rigidly mounted to said output shaft, said chain sprocket having a pitch diameter substantially equal to the axial separation of said pair of cylinders; a chain mounted over said sprocket, opposite ends of said chain being coupled to adjacent ends of said pair of pistons; a

closure member fluid-tightly sealing across said second cylinder portions; and control means for controlling the rotational motion of said output shaft, said control means applying actuating fluid under pressure simultaneously to said chamber and a selected one of said second portions while the nonselected sec-nd portion is being communicated with a low pressure area, said control means also including switching means for switching the selected and then nonselected second portion for reverse motion of said output shaft.

2. A rotary actuator device, characterized by: a body having walls defining a chamber; an output shaft rotatably supported by said body Walls with a portion of the shaft in the chamber; first and second cylinder each having a piston slidable axially therein; means connecting one end of each cylinder to the body with the chamber thereof in communication with said end of the cylinder; end closure means on the opposite end of each cylinder; means providing a positive unidirectional driving connection between the output shaft .and the piston in said first cylinder by which the output shaft is rotated in one direction in consequence of movement of said piston in a direction away from the body, comprising a flexible tension member having opposite ends connected to said pistons at the chamber sides thereof, and having its intermediate portion in said chamber and drivingly engaged with a part on said portion of the output shaft, said tension member being operable upon movement of either piston away from the body to constrain the other piston to move therewith toward the body; and means providing for the simultaneous delivery of pressurized actuating fluid into both ends of said second cylinder and into the body connected end of said first cylinder, together with the exhaust of fluid from the opposite end of the first cylinder to thereby effect motion of the piston therein away from the body and to correspondingly effect rotation of the output shaft in said one direction.

3. The rotary actuator device of claim 2, wherein said last named means provides for the simultaneous delivery of pressurized actuating fluid into the body connected end of a selected one of either of said cylinders and into both ends of a nonselected cylinder, together with the exhaust of fluid from said opposite end of the selected cylinder; and wherein said flexible tension member provides a positive unidirectional driving connection between each piston and the output shaft by which the latter can be driven in either direction of rotation, depending upon which of the pistons is driven toward the exhausting end of its cylinder.

4. The rotary actuator device of claim 3, wherein said fluid delivery means comprises a pair of ports, one for each cylinder and opening to said opposite end thereof to provide for flow of pressure fluid thereto and for exhaust of fluid therefrom; and other port means connecting with said chamber for communicating the same with a source of fluid under pressure.

5. The rotary actuator device of claim 4, wherein said pair of ports communicate only with said opposite ends of their respective cylinders; and wherein said other port means comprises a single port through which pressure fluid can be supplied to said chamber in by-pass relation to the ports of said pair thereof.

6. The rotary actuator device of claim 3, further characterized by a fluid pressure operated shuttle valve having a pair of input ports respectively connected in fluid transfer relation with said opposite ends of the cylinders, and having an output port communicated with the body connected ends of both cylinders and selectively communicable with said input ports to supply pressure fluid to the body connected ends of the cylinders only via the input port communicating with said opposite end of the nonselected cylinder.

7. The rotary actuator device of claim 3, wherein said last named means includes a control valve having a pair of output ports respectively communicated with said opposite ends of the cylinders, and a shiftable valve element operable to selectively communicate said opposite end of either cylinder with a pressure fluid source while concurrently communicating said opposite end of the other cylinder with an exhaust outlet in the valve.

8. The rotary actuator device of claim 3, wherein the body connected ends of the cylinders are commonly supplied with pressurized actuating fluid from said chamber, and further characterized by a pressure operated shuttle valve having a pair of input ports, one for each cylinder and connected in fluid transfer relation with said opposite end thereof, an output port communicated with said chamber to deliver pressure fluid thereto from either input port that is in communication with said opposite end of a non-selected cylinder, and a valve member which closes off the output port from the input port communicating with said opposite end of the selected cylinder.

9. The rotary actuator device of claim 8, wherein said shuttle valve cooperates with a control valve to govern flow of fluid to and from the cylinders, said control valve having an inlet through which pressurized fluid from a source thereof can flow to either cylinder, an outlet through which fluid can be exhausted from either cylinder, and a movable valve element to selectively communicate said inlet thereof with said opposite end of either cylinder while concurrently communicating said opposite end of the other cylinder with the outlet of the valve.

10. The rotary actuator device of claim 2, wherein said last named means comprises a control valve having an inlet connectable with a fluid pressure source, an outlet from which fluid can be exhausted, and a valve element movable from one to the other of a pair of operating positions to selectively communicate its inlet with said opposite end of either cylinder while communicating its outlet with said opposite end of the other cylinder; and wherein said body connected ends of both cylinders are supplied with pressure fluid from said chamber by means communicating the latter with the pressure fluid source in bypass relation to the control valve.

11. The rotary actuator device of claim 3, wherein said part on the output shaft is a sprocket, and the flexible tension member is a chain drivingly engaged with said sprocket; and wherein said cylinders are connected to the body in side by side parallel relation and have their axes tangent to the pitch diameterof the sprocket at opposite sides thereof.

References Cited by the Examiner UNITED STATES PATENTS 1,696,044 12/ 1928 Kuskin 92137 1,756,910 4/1930 Fuller 91172 2,958,197 11/1960 Elliott 121-117.1

FOREIGN PATENTS 484 2/ 1877 Great Britain. 27,232 12/1908 Great Britain. 484,883 9/1953 Italy.

EDGAR W. GEOGHEGAN, Primary Examiner.

RICHARD B. WILKINSON, FRED E. ENGEL- THALER, Examiners.

J. LABOWSKI, P. MASLOUSKY,

Assistant Examiners. 

1. A ROTARY ACTUATOR DEVICE COMPRISING: A PAIR OF PARALLEL CYLINDERS; A FLUID-TIGHTLY SLIDABLE PISTON DISPOSED IN EACH CYLINDER AND SEPARATING EACH CYLINDER INTO A FIRST AND A SECOND CYLINDER PORTION; A CHAMBER FLUID-TIGHTLY SEALED ACROSS SAID FIRST CYLINDER PORTIONS AND PROVIDING COMMUNICATION THEEBETWEEN; AN OUTPUT SHAFT ROTATABLY MOUNTED IN SAID CHAMBER; A CHIN SPROCKET WITHIN SAID CHAMBER AND RIGIDLY MOUNTED TO SAID OUTPUT SHAFT, SAID CHAIN SPROCKET HAVING A PITCH DIAMETER SUBSTANTIALLY EQUAL TO THE AXIAL SEPARATION OF SAID PAIR OF CYLINDERS; A CHAIN MOUNTED OVER SAID SPROCKET, OPPOSITE ENDS OF SAID CHAIN BEING COUPLED TO ADJACENT ENDS OF SAID PAIR OF PISTONS; A CLOSURE MEMBER FLUID-TIGHTLY SEALING ACROSS SAID SECOND CYLINDER PORTIONS; AND CONTROL MEANS FOR CONTROLLING THE ROTATIONAL MOTION OF SAID OUTPUT SHAFT, SAID CONTROL MEANS APPLYING ACTUATING FLUID UNDER PRESSURE SIMULTANEOUSLY TO SAID CHAMBER AND A SELECTED ONE OF SAID SECOND PORTIONS WHILE THE NONSELECTED SECOND PORTION IS BEING COMMUNICATED WITH A LOW PRESSURE AREA, SAID CONTROL MEANS ALSO INCLUDING SWITCHING MEANS FOR SWITCHING THE SELECTED AND THEN NONSELECTED SECOND PORTION FOR REVERSE MOTION OF SAID OUTPUT SHAFT. 